I have an Intel DG965RY motherboard and its specification says it supports 8GB with 533 or 667Mhz RAM sticks and only 4GB with 800Mhz RAM sticks. I am running a 64bit OS.

I earlier had 2 X 1GB sticks (800Mhz), so I bought 2 X 2GB sticks (800Mhz) and I underclocked them in the settings to run at 667Mhz. Shouldn't it support all 6GB RAM now?

It would be a bummer if I will specifically need 667Mhz sticks thinking that at the worst they will underclock and then run at 667Mhz. I tried this because I saw someone posted at some forum that he put in 4GB+ of RAM in the same board @ 800Mhz and the system uses it all.

In my case (On Ubuntu), it only shows 3.2GB as of now (link to Question) so needed to confirm if this is a hardware limitation.

@rakslice sorry didn't get your point. Its the max memory support that's dependent on memory type (frequency). It can obviously run 2GB DIMMs. Did I miss anything? Can you quote the specific line you are referring to?
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AshfameNov 27 '11 at 8:15

Have you looked for a BIOS update ? Your system is behaving like 32-bit instead of 64-bit. It may also be because of some obscure BIOS option, but I cannot find details on your BIOS : Do you have a link to it ?
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harrymcNov 27 '11 at 9:05

I'm not sure what that means, but I see there's a confusing tradition of describing dual rank memory as "dual-sided" or as having a lot of chips (http://en.wikipedia.org/wiki/DIMM#Ranking).

'JEDEC decided that the terms "dual-sided," "double-sided," or "dual-banked" were not correct when applied to registered DIMMs.' Oooh, tantalizingly close to just putting a consistent naming standard right into the spec.

Regardless of what that means, where did you saw that the Nanya RAM sticks are Double-sided DIMMs with x16 organization? We can atleast narrow it down to sticks being incompatible.
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AshfameNov 27 '11 at 9:54

Is it this line under Base components? 128M x 8 / 16 Also shouldn't they be totally incompatible or show half RAM or something? I mean why would it show 3.2GB when 4GB of incompatible RAM is there.
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AshfameNov 27 '11 at 9:57

Support for up to 8 GB of system memory using DDR2 667 or DDR2 533 DIMMs

Support for up to 4 GB of system memory using DDR2 800 DIMMs

So yes, this is a hardware limitation :
Since all your sticks are at 800Mhz, then your usable RAM is limited to 4 GB,
which is what you are seeing.
The 3.2GB are probably what is left after device memory was allocated.

I see no other solution (apart from changing the motherboard),
except underclocking to 667Mhz some or all of the sticks.
The manual is unclear as regarding the mixing of RAM sticks,
and my understanding is that you only underclocked the 2GB sticks while leaving the 1GB at 800Mhz,
so you might need to underclock all of them.

Note: By "underclocking" I mean getting 667Mhz RAM sticks, rather than lowering the frequency through the BIOS. Going by your screenshot, changing the frequency does not still prevent the BIOS from detecting the sticks as being 800Mhz in nature.

+1 for sounds plausible - particularly the "might need to underclock all of them".
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Steve314Nov 27 '11 at 11:04

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Yes, that's what I wrote in the question. Did you check the screenshot? I have set the frequency to 667Mhz. Shouldn't that underclock all the 4 sticks? I see the clock reduced to 667Mhz when I see it through lshw
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AshfameNov 27 '11 at 11:13

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Look guy, you have your logic but you came here because it doesn't work, so you might listen instead of undervoting answers. I am not at all sure that setting the frequency to 667Mhz somehow magically converts all the DIMMs to 667Mhz. My opinion is, as they are still coming up as 800 in your screenshot, that the motherboard makes its decision based on the DIMM type rather than on the frequency, so you have only 4GB. In other words: You didn't buy the right RAM sticks to have 6GB.
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harrymcNov 27 '11 at 12:14

Did I opposed your suggestion? No! I said what makes sense to me and asked a question in return to clarify. Whats wrong in that? I am OK with not having 6GB, I just need to clarify if it can be fixed. I understand they still might be treated as 800Mhz DIMMs but then option of setting a frequency should have a purpose. No? Lastly, I don't upvote/downvote any answer unless an issue is resolved. This one is pending.
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AshfameNov 27 '11 at 12:54

My apologies - I thought the downvoter was you. The frequency surely has its desired effect, but apparently the BIOS decides on the maximal RAM based rather on the electronics it encounters inside the socket.
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harrymcNov 27 '11 at 13:01